On Thu, 19 Nov 2015 01:10:27 -0000
wrote:

Ralph Mowery wrote:

"Steve" wrote in message
news:20151119002316.0a9bd48f@silent...
With a RTL SDR type USB stick on its way, I turned my attention to
looking for an antenna with high gain that would give me good
coverage of ADS-B 1090 MHz broadcasts from aircraft, one that I
could easily and cheaply make for myself.

A quick google produced loads of hits for this type of antenna.

https://www.youtube.com/watch?v=TkUYdCPFXXs

as a random one. You get the idea.

They all consist of 'accurately' cut and calculated equal elements
that reverse the phase 180 degrees for each element so the theory
goes.

Pondering over this, it struck me that although the websites do
take into account velocity factor of the coax, that is for the
coax in normal operation. Once the outer conductor, or screen
becomes an element in an array, exposed to the outside world it
has a faster velocity of propagation which, in turn means that the
outer of the coax must be longer than the inner which is there to
maintain phase on each segment. In other words an impossible
antenna to make.

Those were my thoughts which have led me to have doubts about the
cheap, simple designs that abound.

Any thoughts, or have I missed something?

Steve G8IZY

You are really missing 2 things. For the coax sections, when using
the outer sections, they are the actual antenna elements and the
iner conductor are not used. Then the sections in between are
using the velocity factor of the coax for the phasing sections.

That means (if using 1/4 wave sections) that every other section
will be almost 1/4 wave without any velocity factor correction
(actually a very small one) and the other sections will be 1/4 wave
times the .66 velocity factor of the coax (or whatever the VF is
for that particular coax.


The other thing, to get gain the antenna pattern is compressed so
the signal will be greater toward the horizon and not so much up in
the air where the planes are. Two or 3 elements may be ok,but
going to a large number may not work as well for the planes.

The problem is similar to non-tracking satellite antennas.

The source will be farthest away when it is close to the horizon, but
a lot of sources will be much higher in elevation so you do need some
gain in the upward direction.

Looking at existing 70 cm satellite antennas might be worthwhile.


Nope! I am looking for maximum gain to the horizon, like any DXer, for
the reason of being able to identify good and bad directions from here,
using the statistical tools available.

I've had loads of advice about watching aircraft near me or overhead
which is not what I wanted as if I want to track a plane, I'll look at
Planefinder or FR24 if my 'gain' antenna lets me down!